Heat transfer from waste gas of a cement kiln to pulverulent raw material



y 28, 1968 R. RUEGG 3,385,580

HEAT TRANSFER FROM WASTE GAS OF A CEMENT KILN TO PULVERULENT RAWMATERIAL Original Filed Aug. 14, 1964 2 Sheets-Sheet 1 INVENTOR. RU DOLFRU E66 ATTORNEYS May 28, 1968 R. RUEGG 85 80" HEAT TRANSFER FROM WASTEGAS OF A CEMENT KILN TO PULVERULENT RAW MATERIAL Original Filed Aug. 14,1964 2 Sheets-Sheet 2 INVENTOR.

RUDOLF RUE-I66 ATTO RNEYS United States Patent Oftice s 11 Claims. Zci.263-32) This is a continuation of application Ser. No. 389,668 filedAug. 14, 1967, now abandoned.

This invention relates to a process for the heat transfer from a wastegas stream issuing from a cement kiln, to pulverulent raw material to besupplied to the cement kiln, and to an apparatus for carrying out saidprocess.

It is known to pre-heat pulverulent raw material in open heat exchangeby the waste gases of a rotary kiln. In a known apparatus, the wastegases fiow in cross current through the granulated raw material chargedonto a travelling grate, which involves a high heat consumption andconsiderable equipment outlay. It is furthermore known to admix rawmaterial with waste gases, preferably in one or more cyclones connectedin series, and to separate them again. In such plants, clogging ofheat-exchangers and conduits carrying material readily occurs inconsequence of caking and incrustation of raw material, and this leadsto irregularities and trouble in operating the plant.

It is the aim of this invention to overcome these drawbacks. For thispurpose, a process of the kind hereinbefore described is carried outaccording to the invention by dividing the raw material stream into acoarse fraction and a fine fraction and then allowing of the twosubdivided streams, only the coarse fraction to participate in the heatexchange, the heat exchange between the waste gases and the materialbeing eifected in one or more zones of the gas stream.

The invention is based on the knowledge that predominantly the finefractions of the raw material clog in the heat-exchangers and conduits.The tendency of the raw material to clog increases with its alkalicontent, since the alkali vapours coming from the rotary kiln arepreferably deposited on the fine fractions and the latter thereby becomesticky. The chlorine content of the fuel leads to the formation ofalkali chlorides, which additionally promote clogging.

In carrying out the process according to the invention of dividing theraw material into its fine fraction and coarse fraction and of allowingthe latter only to participate in the heat exchange, it is sufiicient toseparate out approximately to percent of the total raw material as finefraction to prevent the occurrence of appreciable clogging with thecoarse fraction thus remaining. The fine fraction separated out to saidextent has such a high heat transfer coefficient in consequence of theratio of surface to volume that its introduction into the rotary kiln inthe cold state does not substantially effect the combustion process. Ifnecessary, the still undivided raw material may also take part in theheat exchange in a comparatively short action, in which the finefractions are adequately heated. The subdivision into the fine fraction,passing directly to the rotary kiln, and into coarse fraction,participating in the heat exchange, occurs in this case in a region, inwhich the raw material stream has already absorbed heat from the wastegases.

A further step according to the invention for preventing clogging maycomprise removing from the waste gases the alkali vapours coming fromthe rotary kiln. This step,

3,3355% Patented May 28, 1968 at the same time, prevents the continuousconcentration in alkali content which occurs in the plant in consequenceof the open heat exchange, and which has a detrimental effect on thecombustion process and the quality of the produced cement clinker.Since, for given raw material and given fuel, a number of alkalicompounds having dilferent precipitation temperatures are produced, theseparation of the very fine dust contained in the waste-gas stream andon which the alkali vapours are condensed will be carried out insuitable temperature ranges of the heatexchange section, and in this wayall the alkali compounds will be covered, thereby affording the furtheradvantage that high-alkali dust is obtained separately from lowalkalidust, which is separated in the final dust arrester, usually necessaryin any event for keeping the air clean, and at all events in othertemperature ranges. The heatexchange zones are so designed that thewaste-gas stream flowing separately from the material has at one or morepoints the temperature necessary for the condensation of the alkalivapours.

Constructional examples of apparatus for carrying out the processaccording to the invention are represented in simplified form in theaccompanying drawings, with reference to which the process will beexplained. In these drawings:

FIG. 1 shows a diagram of a plant for pre-heating raw cement flour bythe waste gases of a rotary kiln, and

FIG. 2 is a diagram of a plant of another embodiment, correspondingparts having the same references.

The plant shown in FIG. 1 has a rotary kiln 1 of known type, into whichopens a fine-fraction conduit 2, coming from the fine-fraction dischargeconnection of a classifier 3 of the cyclone type. The classifier 3 maybe one of the type, through which flows an additional gas stream, thematerial to be classified being in open heat exchange with theadditional gas. The coarse fraction discharge connection of theclassifier 3 is connected by a coarse fraction pipe having sections 4,5, 6 to the rotary kiln 1, cyclones 7, 8 being connected in series inthe coarse fraction pipe having the sections 4, 5, 6, and a waste-gaspipe, having the sections 9, 10, 11 leading from the rotary kiln '1 tothe additional gas inlet connection of the classifier 3, each of saidcyclones 8, 7 having a common inlet connection for receiving theincoming coarse fraction of the raw material and the waste-gas stream,while the raw material and the waste gas are discharged through separateconnections. Leading from the additional gas discharge connection of theclassifier 3 is a waste-gas pipe having sections 12, 13, in which areinserted a final dust arrester 14 and an air exhauster 15 at the foot ofa chimney 16.

Connected in the section 9 of the waste-gas pipe is a distributing valve17, from which a bypass pipe 18 branches, a multicyclone 19 beingconnected in said bypass pipe 18. Distributing valve 17, bypass pipe 18and multicyclone 19 may be so dimensioned that the quantity of waste gascoming from the rotary kiln 1, about 25 percent can be passed throughthe multicyclone 19 instead of direct to the cyclone 8. Branching fromthe chimney-side part of the section 13 of the waste-gas pipe is a coldgas pipe 20, leading to a mixing valve 21 inserted in the part of thebypass pipe 18 situated between the distributing valve 17 and the inletconnection of the multicyclone 19. Built into section 11 of thewaste-gas pipe is a changeover valve 22, the branch connection of whichopens through a bypass pipe 23 into the section 12 of the waste-gaspipe.

The plant operates as follows. The raw material is introduced into theclassifier 3, where it is divided into a fine fraction and a coarsefraction, the fine fraction conveniently amounting to from 10 to 2.0percent of the total quantity of coarse material. The fine fractionpasses through the fine fraction pipe 2 into the rotary kiln 1, and thecoarse fraction passes through the coarse fraction pipe with thesections 4, 5, 6 into the rotary kiln 1. An open heat exchange takesplace in two zones comprising the cyclones 7, 8, respectively, which areinserted in the coarse fraction pipe and through which the waste gasesfrom the rotary kiln 1 fiow at the same time. In the position of thechange-over valve 22 shown in the drawing, the waste gases from thecyclone 7 enter a further heat exchange zone which comprises theclassifier 3, where an open heat exchange occurs between raw materialand waste gases. The waste gases leave the classifier 3 via section 12of the waste-gas pipe, are purified in the final dust arrester 14 andare blown into the chimney through section 13 of the Waste-gas pipe bythe air xhauster 15. Thus, no fine fraction which would adhere fast onthe walls of the material pipes and cyclones enters the heat-exchangezones comprising the cyclones 7, 8, but after a slight heat exchange,which however is suirlcient in view of their high coefiicient of heattransfer, the fine fraction of the raw material bypasses the cyclones 7and 8 and enters directly the rotary kiln 1. Only the coarse fraction,which is much less prone to clogging the parts of the plant throughwhich it flows, passes through the heat-exchange zone comprising thecyclones 7, 8, in which heat exchange between waste gases and materialprincipaL ly takes place.

If there is a danger of clogging the classifier 3 by the fine fractionheated by the waste gases, then by throwing over the change-over valve22 to the position not shown in the drawing, the waste gas stream couldbypass the classifier 3 and pass directly to the final dust arrester 14.

With the described process it is thus possible to avoid clogging due tocaking and incrustration of material, and the working reliability andservice period of the plant can be considerably improved.

The waste gases from the rotary kiln 1 contain very fine dust and alkalivapours. By means of the distributing valve 17 at least approximately 75percent of the quantity of waste gas produced at any time can be passeddirectly to the cyclone 8, and at the most approximately 25 percent canbe passed previously through the multicyclone 19, Where the very finedust, with the alkalis precipitated on it, is separated from the partialstream of the waste gases, it being possible by adding comparativelycold waste gases from pipe 20, 'by means of the mixing valve 21, to thewaste-gas partial stream in the bypass pipe 18, to ensure that theWaste-gas partial stream has the optimum temperature for precipitationof the alkalis. The chlorine-containing alkali compounds promotingclogging of heat-exchangers and pipes carrying material to a veryparticular extent are preferably precipitated at the high temperaturesprevailing in the waste gas passage between the rotary kiln 1 and thefirst heat exchange zone comprising cyclone 8, so that the precipitationof these alkali compounds alone substantially improves the workingreliability and service period of th plant. According to experienece,removal of the highalkali dust from approximately 25 percent of theWaste gas stream in the case of high-chlorine alkali compounds and fromapproximately 10 percent of the waste gas stream in the case oflow-chlorine alkali compounds is sufficient to ensure trouble-freeoperation during a long service period.

The plant shown in FIG. 2 affords a further improvement. In addition tothe parts provided in the plant shown in FIG. 1, distributing valves 24,28 are also inserted in the sections 10, 11 of the waste-gas pipesituated between the heat exchange zones comprising the cyclones 8 and7, respectively, and between the heat exchange zone comprising thecyclone 7 and the heat exchange zone comprising the classifier 3.Branching off the said distributing valves are bypass pipes 25, 29, inwhich are inserted multicyclones 26, 30, mixing valves 27, 31, connectedto the cold gas conduit 2t), being connected in the parts of the bypasspipes 25, 29 situated between the distributing valves 24, 28 and theinlet connections of the multicy clones 26, 30, and the distributingvalves 17, 24, 28 being so dimensioned that the entire waste gasquantity can be passed through each multicyclone 19, 26, 30.

The plant shown is based on the assumption that the alkali vapours fromthe rotary kiln are precipitated substantially in three, not too Widetemperature ranges. Suitable dimensioning of the heat-exchange zonesensures that the waste gas stream flowing separately from the stream ofmaterial in sections 9, 1t), 11 of the waste-gas flow passage hassubstantially the temperature necessary for precipitaion of the alkalivapours, it being possible to carry out fine correction by means of themixing valves 21, 27, 31 in the manner described.

Even when practically all the very fine dust coming from the rotary kiln1 with the waste gases is separated from the waste gas stream by themulticyclone 19, and practically dust-free waste gases enter the cyclone8, very time dust is again produced in the cyclone 8 from the coarsefractions, so that very fine dust is again present in the waste gasespassing from the cyclone 8 to section 10 of the waste-gas pipe.

At the temperatures prevailing in this section 10 of the waste gas flowpassage, alkalis are precipitated on the said very fine dust, and thevery fine high-alkali dust is separated from the waste-gas stream inmulticyclone 26. -In the same way, the Waste gases leaving cyclone 7contain very fine dust, on which are precipitated the alkalis which arecondensable at the temperatures prevailing in this section l ll of theWaste-gas flow passage.

With the use of raw material and/or =fuel producing alkalis which areprecipitated only on two or one temperature range, one or twomulticyclones may be bypassed, or however the low-alkali dust separatedby them may be re-supplied to the kiln. It is also possible to allowonly part of the waste-gas stream to flow through one or moremulticyclones. Likewise, the multicyclones 19, 126, 30 could be sodimensioned that only part of of the waste-gas stream can be passedthrough them.

When using raw material and/or fuel other than those on which thedimensioning of the heat-exchange Zones has been based, there may beshifting of the temperature ranges, so that the temperatures, at whichthe alkali vapours are precipitated, are outside the sections 9, 10, illof the waste-gas flow passage and inside the heat exchange zonescomprising the cyclones 8 and 7 and the classifier 3. If thetemperature-range shifts are uniform, the necessary correction can bemade by varying the ratio of the charged raw material quantity to thefuel fired. In the case of non-uniform and/or only partial shift of thetemperature ranges, by varying the ratio of the raw material to fuel, arange advantageously the hottest, is brought into the correspondingsection of the waste-gas flow passage and the position of the otherranges is corrected by the addition of cold gas. During operation of theplant, the position of the temperature ranges Within the plant can beadjusted by continuous supervision of the alkali content of the veryfine dust separated by the multicyclones 19, 26, 30.

The described steps permit separation from the waste gases of a largepart of the alkali compounds produced, and prevent concentration of thealkali content in the plant. A further advantage is that very finehigh-alkali dust is produced separately from very fine low-alkali dust.

I claim:

1. A process for the transfer of heat from the hot waste gases issuingfrom a cement kiln to the pulverulent raw material being supplied tosaid kiln comprising (a) passing the raw material through a classifierand separating the raw material into a coarse fraction and a linefraction;

(b) forming a flowing stream from said waste gases;

(c) entraining the separated coarse fraction in said stream so that thecoarse fraction is heated thereby;

(d) separating the entrained heated coarse fraction from said stream;

(e) preventing heat exchange between said separated fine fraction andsaid stream; and

(-f) supplying the heated coarse fraction and the fine fraction to saidkiln.

2. The process defined in claim 1 in which the p'ulverulent raw materialis also brought into heat exchange contact with waste gas issuing fromthe cement kiln before the separation of said fine fraction.

B. The process defined in claim -1 in which at least part of the wastegas stream, after its separation from the heated coarse raw material, isbrought in heat exchange contact with the raw material before theseparation of the fine fraction.

4. In the process defined in claim 1 separating fine dust particles,which may be present in the waste gas stream, from at least a portion ofsaid waste gas stream and in a temperature range in which alkali vaporsoccurring in said portion of said waste gas stream may condense on saidfine particles, said fine dust separation being accomplished before saidentrainment of the coarse fraction in said waste gas stream.

5. An arrangement for the heat transfer from a waste gas stream issuingfrom a cement kiln to pulverulent raw material to be supplied to saidcement kiln, comprising a classifier for the separation of a finefraction of said raw material from a remaining coarse fraction of theraw material, said classifier having a supply connection for unheatedraw material and two discharge connections, one for the supply of saidseparated fine fraction to the cement kiln and the other for thedischarge of the remaining coarse fraction of the raw material; meansdefining a flow passage for said waste gas stream, including at leastone heat exchange zone having an inlet for the coarse fraction to beentrained in the waste gas stream and a cyclone at its outlet forseparating the entrained coarse fraction from the waste gas stream; andmeans for leading the coarse raw material fraction discharging from saidcyclone to the cement kiln.

6. The arrangement defined in claim 5 in which said waste gas flowpassage includes at least two serially arranged heat exchange zones,each having a coarse material inlet for entraining said coarse materialwith the waste gas and a cyclone for separating said entrained coarsewaterial from the gas, and in which means are provided for seriallyleading the coarse raw material fraction through said heat exchangezones.

7. The arrangement defined in claim 5 in which the flow passage of theWaste gas stream opens into the classifier so as to admix at least partof the waste gas leaving said heat exchange zone with the raw materialsupplied to the classifier before the separation of the fine fraction.

8. The arrangement defined in claim 5 in which means are provided forseparating fine dust from at least part of the waste gas stream in atleast one point of the waste gas flow passage situated outside said heatexchange zone.

9. The arrangement defined in claim 8 in which adjustable cold gassupply means are provided in advance of the fine dust separating means.

10. An arrangement for the transfer of heat from a waste gas streamissuing from a cement kiln, to pulverulent raw material consisting of acoarse fraction and a fine fraction, to be supplied to said cement kiln,com- ,prising means defining a fiow passage for said waste gas streamincluding serially arranged heat exchange zones;

' means for leading the coarse fraction raw material serially througheach of said heat exchange zones of the gas fiow passage before itsbeing supplied to the cement kiln,

the material being introduced into the gas stream at the upstream end ofeach heat exchange zone; means separating the coarse fraction from thegas at the downstream end of each respective zone; and means causing thefine fraction of the raw material to bypass at least the heat exchangezone which is the first in the direction of flow of the waste gas.

' 11. The arrangement defined in claim :10 which comprises fine dustseparating means in said gas flow passage in at least one point situatedbetween said heat exchange zones.

References Cited UNITED STATES PATENTS 2,590,090 3/1952 De Vaney 263-322,785,886 3/1957 Muller 263 32 3,212,764 10/1965 Muller et al. 263-42FREDERICK L. MAT-'IESON, JR., Primary Examiner.

JOHN J. CAMBY, CHARLES J. MYHRE,

Examiners.

1. A PROCESS FOR THE TRANSFER OF HEAT FROM THE HOT WASTE GASES ISSUINGFROM A CEMENT KILN TO THE PULVERULENT RAW MATERIAL BEING SUPPLIED TOSAID KILN COMPRISING (A) PASSING THE RAW MATERIAL THROUGH A CLASSIFIERAND SEPARATING THE RAW MATERIAL INTO A COARSE FRACTION AND A FINEFRACTION; (B) FORMING A FLOWING STREAM FROM SAID WASTE GASES; (C)ENTRAINING THE SEPARATED COARSE FRACTION IN SAID STREAM SO THAT THECOARSE FRACTION IS HEATED THEREBY;